The variation of microstructure and microtexture across a Ti-6Al-4V (Ti64) linear friction weld was investigated using scanning electron microscopy (SEM) and electron backscattered diffraction ...(EBSD). Pole figures and misorientations distribution obtained from α phase show four distinct regions within the weld, with different textural characteristics. Nevertheless, the main texture components remain the same in centre weld zone and thermal mechanical affected zone with the basal poles of the α phases located at around 0°, 60° and 90° to the sample normal (Z0) direction. The results indicate that the texture components are strongly related to the amount of transformed β phase. The deformation of the primary α grains has a limited effect on the texture development.
Micro-bending tests were performed on single-crystalline cantilever beams of long-period stacking ordered (LPSO) structures of Mg85Zn6Y9 alloy with and without a notch to study crystal plasticity and ...cracking behaviour under basal and non-basal slip conditions. The activation of basal slip induced load drops during the deformation of a plain cantilever beam with the bending moment perpendicular to the basal plane. A cantilever sample oriented for prismatic slip exhibited high yield stress and moderate strain hardening. The apparent critical resolved shear stress (CRSS) for prismatic slip was determined to be ∼360 MPa, significantly higher than 30 MPa CRSS determined for basal slip. A crystal plasticity finite element analysis of the micro-bending test on the sample oriented for prismatic slip was performed to verify the CRSS values determined. In addition, the intrinsic fracture resistance for crack growth along the direction on a prismatic plane is higher than that for crack growth along the direction on a basal plane. In combination, these results indicate that inhibition of basal slip activity contributes not only to high strength but also to high intrinsic toughness observed in this LPSO structure.
•We studied crystal plasticity and cracking in basal and non-basal slip conditions.•Micro-bending tests were performed on single crystals of Mg85Zn6Y9 LPSO structure.•Sample oriented for prismatic slip showed high yield stress and moderate hardening.•Critical resolved shear stress for the prismatic slip was determined ∼360 MPa.•Prismatic slip contributes not only to high strength but to high intrinsic toughness.
The quest to produce fully dense nanostructured ceramics has received much attention over the past 10 years. This short review highlights some of the progress made and indicates some of the avenues ...where further research should prove fruitful. The review concentrates mainly on the attempts to produce nanograined dense ceramics from transition aluminas, where the phase transformation into the thermodynamically stable crystallographic alpha alumina just above 1000 °C plays an important role. The review also restricts itself to traditional low pressure room temperature forming methods and sintering cycles without any applied stress. Highlights for other systems such as yttria and zirconia where dense nanograined ceramics have been successfully produced will also be described and discussed.
In this article we shall describe our quest and ultimate success in furthering our understanding of the action of superplasticizers on the rheology of cement and concrete. By specifically producing ...superplasticizers with varied architectures, we have been able to show the important structural features of the macromolecules that lead to a successful superplasticizer or water reducing agent. Both polycarboxylate and lignosulfonate polymers have been investigated. Using both non-reactive model MgO powders, three different types of cement blends, the adsorption behaviour and the effect on the rheological properties of these two important superplasticizer families have been used to further develop a conceptual model for superplasticizer — cement behaviour. This paper will deal mainly with the conceptual model, the materials and methods used to asses the polymer adsorption behaviour and rheological properties of the systems studied. We shall briefly describe the adsorption of the polymers onto the different surfaces and their influence on surface charge and rheology and the influence of the various ionic species found in cement pore solutions that may influence polymer-cement affinity. The key factors are shown to be the effective adsorbed polymer thickness and the induced surface charge which can be influenced by the polymer architecture, the pore solution composition and the initial particle surface charge.
Approximations in the moist thermodynamics of atmospheric models can often be inconsistent; different parts of numerical models may handle the thermodynamics in different ways, or the approximations ...may disagree with the laws of thermodynamics. To address these problems, all relevant thermodynamic quantities may be derived from a defined thermodynamic potential; approximations are instead made to the potential itself—this guarantees self‐consistency, as well as flexibility. Previous work showed that this concept is viable for vapour and liquid water mixtures in a moist atmospheric system using the Gibbs potential. However, on extension to include the ice phase, an ambiguity is encountered at the triple point. To resolve this ambiguity, here the internal energy is used instead. Constrained maximisation methods on the entropy can then be used to solve for the system equilibrium state. Nevertheless, a further extension is necessary for realistic atmospheric systems, where many important non‐equilibrium processes take place; for example, freezing of supercooled water and evaporation into subsaturated air. To capture processes such as these fully, the equilibrium method must be reformulated to involve finite rates of approach towards equilibrium. Here the principles of non‐equilibrium thermodynamics are used, beginning with a set of phenomenological equations, to show how non‐equilibrium moist processes may be coupled to a semi‐implicit semi‐Lagrangian dynamical core. Standard bubble test cases and simulations of idealised cloudy thermals are presented to demonstrate the viability of the approach for the equilibrium regime. Further details and results for non‐equilibrium regimes are presented in Part II.
The moist thermodynamics of atmospheric models is not always treated consistently, leading to possible errors in the energy budget. To improve consistency, we derive all thermodynamic quantities of interest from the internal energy potential, and develop a framework to capture non‐equilibrium thermodynamic processes in a dry‐vapour–liquid‐ice atmospheric system. This system may then be coupled to a semi‐implicit semi‐Lagrangian dynamical core and standard bubble tests carried out, as in the accompanying graphic.
In numerical models of the atmosphere, the non‐equilibrium thermodynamic processes involving moisture are not always treated consistently—possibly leading to inconsistencies and errors in the energy ...budget. Therefore, a more consistent formulation of (moist) thermodynamics is important, for short‐timescale weather models and long‐timescale climate models. In Part I of this work, we derived a thermodynamically consistent framework, describing condensation, evaporation, freezing, and melting of cloud droplets, in which all thermodynamic quantities of interest were derived from an internal energy potential and with the moist thermodynamics coupled to a 2D semi‐implicit semi‐Lagrangian dynamical core. While this framework was primed to express non‐equilibrium processes, it was solved for the equilibrium regime only. Here, we follow the methods in Part I, but with the expression for the non‐equilibrium processes “turned on”, for example, allowing freezing of supercooled water or evaporation into subsaturated air. To implement the proposed approach, it is necessary to translate conventional atmospheric microphysics expressions for transfer rates of matter and entropy in and around a cloud droplet into the formalism of non‐equilibrium thermodynamics. This procedure is first derived for some simple idealised cases, beginning with liquid droplet growth by vapour diffusion, and proceeding to more complex three‐phase cases. To demonstrate the approach, we then simulate some idealised cloudy thermals, comparing the equilibrium and non‐equilibrium regimes—finding a robust decrease in the vertical velocity in the non‐equilibrium regime, as expected. Thus, this work demonstrates the feasibility of building a numerical model that includes a framework for modelling the moist non‐equilibrium thermodynamics of an atmospheric system consistently and provides a step towards this type of more consistent atmospheric modelling.
In Part I, a more thermodynamically consistent framework was developed to represent the moist non‐equilibrium thermodynamics of an atmospheric system and solve numerically in equilibrium mode. For a complete non‐equilibrium solution, however, standard microphysics formulas must first be translated into the formalism of non‐equilibrium thermodynamics. Once we have done this, we then outline a method to transition between different microphysics cases—as shown in the graphic—and present simulations of idealised cloudy thermals to demonstrate proof of concept.
Understanding and controlling precipitation reactions is a major challenge for industrial crystallization. Calcium carbonate is a widely studied system: more than 3000 papers have been devoted to ...the subject over the past 10 years. The first step of the precipitation of calcium carbonate, from relatively concentrated solutions (0.01 mol/L), involves the formation of an initial gel phase which later transforms into calcite, vaterite, or a mixture of both phases. Our work aimed at controlling this first step. Nanosized seeds (8 nm), formed in situ, were used in order to control the often chaotic nucleation step which normally leads to poor phase selection and broad particle size distributions. Seeding has often been used to avoid spontaneous nucleation in metastable solutions for growth mechanism investigations of single-crystal calcium carbonate. Here the ability of a seeding method to control the precipitation reaction evolution even in the case of high supersaturation is demonstrated. The seeds and the presence of a polymeric additive (poly(acrylic acid)) allow the control of the precipitated polymorph and the specific surface area, while maintaining a narrow particle size distribution in the submicron range. Direct characterization methods did not succeed in identifying these nanoseeds; indirect methods using solubility calculations are used to demonstrate their existence and quantify size and number density of the nanosized seeds.
Fatigue crack growth resistance of an austenitic stainless steel Alloy 709 has been evaluated at temperatures of 550, 650 and 750 °C in air and vacuum. Tests were conducted at a frequency of 0.25 Hz ...and a stress ratio of 0.1. The linear elastic stress intensity factor range (ΔK) has been used to characterise fatigue crack growth resistance. A modest detrimental effect of air at elevated temperatures on fatigue crack growth is identified and discussed. Striated transgranular fatigue is found to be the failure mechanism for all test conditions. The formation of striations and the interaction of crack growth with slip traces are further investigated using in-situ testing (within a scanning electron microscope), together with transmission electron microscopy carried out on samples extracted by focused ion-beam milling perpendicular to fracture surfaces. Finally, an analytical model is proposed to predict fatigue crack growth in air for Alloy 709.
Polycarboxylate molecules and oligomers have been investigated as growth modifiers during seeded calcite precipitation. To better understand possible molecular interactions and kinetic effects, ...additives with different structures and molecular weights have been investigated in this work. All additives show growth modifying effects, albeit less strongly for succinic acid and glutaric acid. This is attributed to a relatively weak interaction with the precipitated particles as well an additive size too small to influence the aggregation phase of the growth mechanism. Poly(acrylic acid) and poly(aspartic acid), on the other hand, led to strong growth modification, with the resulting particles being nanostructured, formed by an assembly of nanosized primary particles and consequently having a high specific surface area. Poly(aspartic acid) showed a stronger growth modifying effect than poly(acrylic acid) at a similar molecular weight and functional group concentration. This was not readily explainable without using molecular dynamics simulations (reported in a separate article), which suggests that the differences originate from the rigidity of the backbone and favorable electrostatic interactions between backbone nitrogen atoms and the surface in the case of poly(aspartic acid).
Short crack phenomena are considered important for lamellar structures in γ-TiAl alloys and have been thoroughly investigated in the past. However, the short cracks in the previous studies were ...nearly all introduced artificially. No particular investigations have looked into the initiation of fatigue short cracks. Therefore, naturally initiated short fatigue cracks at room temperature under two different stress ratios (0.1 and 0.5) were investigated in a near-lamellar γ-TiAl alloy (Ti-45Al-2Mn-2Nb) in this study. The observations show that the fatigue crack initiation behaved differently at low and high stress ratios. At low stress ratio, the specimens failed at lower ultimate stress levels (σmax = 450 and 475 MPa), and the crack initiated from the cluster of interlamellar fracture near mode-I orientation or stress concentration areas. At the higher stress ratio, the specimens failed at higher but consistent stress levels (σmax = 560 and 570 MPa), and in the specimen crack initiation areas, the interlamellar fractures were still the primary fracture mode, whereas some were found at tilted angles due to shear deformation. The results suggest that short fatigue cracks can naturally initiate in lamellar γ-TiAl alloys, thus attention should be paid to their microstructure design, surface finishing and cleanliness.
PDF
